1. Field of the Invention
The present invention generally relates to piezoelectric thin-film resonators and filters, and more particularly, to a piezoelectric thin-film resonator having an upper electrode and a lower electrode overlapping each other and a piezoelectric film interposed between the upper electrode and the lower electrode, and a filter formed with such piezoelectric thin-film resonators.
2. Description of the Related Art
Due to the rapid spread of high-frequency wireless devices such as portable telephone devices, there is an increasing demand for small-sized, lightweight high-frequency filters to be used at high frequencies from 900 MHz to 5 GHz. In such fields, filters formed mainly with surface acoustic wave devices are used. Recently, attention has been drawn to piezoelectric thin-film resonators as devices that exhibit excellent characteristics especially at high frequencies, and can be made smaller in size and turned into monolithic types. Filters that include those piezoelectric thin-film resonators have also been drawing attention.
Examples of piezoelectric thin-film resonators include resonators of a FBAR (Film Bulk Acoustic Resonator) type and a SMR (Solidly Mounted Resonator) type. A piezoelectric thin-film resonator of the FBAR type is disclosed in FIG. 1 of U.S. Pat. No. 6,291,931 (hereinafter referred to as “Patent Document 1”). In this piezoelectric thin-film resonator, an upper electrode and a lower electrode are provided on both sides of a piezoelectric film. The region in which the upper electrode and the lower electrode sandwiching the piezoelectric film face each other is a membrane region. Although not shown in the drawings of Patent Document 1, the lower electrode, the piezoelectric film, and the upper electrode are formed on a substrate, and a void is formed in the substrate immediately below the region in which the lower electrode and the upper electrode face each other. The void is formed by etching the substrate from its bottom face. Alternatively, the void may be formed through the upper surface, using a sacrifice layer. A piezoelectric thin-film resonator of the SMR type is disclosed in FIG. 2 of Patent Document 1. Instead of the void in the substrate, an acoustic reflection film formed by alternately stacking films with high acoustic impedance and films with low acoustic impedance is used, and the film thickness of the acoustic reflection film is ¼ of the wavelength of the elastic wave.
In each of the above piezoelectric thin-film resonators, a high-frequency electric signal is supplied between the upper electrode and the lower electrode, so as to excite an elastic wave in the piezoelectric film by virtue of an inverse piezoelectric effect. At the same time, due to a piezoelectric effect, the distortion caused by the elastic wave is converted into an electric signal. This elastic wave is totally reflected by the faces of the upper electrode and the lower electrode in contact with the air. Resonance is caused at such a frequency that the total film thickness H of the upper electrode, the piezoelectric film, and the lower electrode is an integral multiple (n times) of ½ of the wavelength of the elastic wave. Where the propagation speed of the elastic wave that is determined by the material is represented by V, the resonant frequency F is expressed as: F=nV/2H. In this manner, the resonant frequency is controlled by adjusting the film thicknesses. Thus, a piezoelectric thin-film resonator with desired frequency characteristics can be obtained.
A ladder filter is a bandpass filter that has a predetermined passband region and has piezoelectric thin-film resonators arranged in series arms and parallel arms in a ladder-like fashion.
As a large amount of information is being handled today, high-frequency wireless devices are strongly expected to employ broadband filters. To provide broadband devices, resonators each having a high electromechanical coupling coefficient (k2) are necessary. Therefore, the following conventional techniques have been suggested.
By the first conventional technique, the piezoelectric film is made of lead zirconate titanate (PZT) or lead titanate (PbTiO3), which has a higher electromechanical coupling coefficient than aluminum nitride (AlN) and zinc oxide (ZnO) and is widely used as piezoelectric ceramics (Prior Art 1).
By the second conventional technique, the orientation of the piezoelectric film is improved so as to increase the electromechanical coupling coefficient (Prior Art 2). For example, the relationship between the piezoelectric film orientation and the electromechanical coupling coefficient in a case where the piezoelectric film is made of aluminum nitride is disclosed in “Piezoelectric Materials for BAW Resonators and Filters (H. P. Lobl, et al., pp 807-811, 2001 IEEE Ultrasonics Symposium, IEEE, the United States)”.
By the third conventional technique, the electromechanical coefficient is increased by controlling the film thickness ratio of the piezoelectric film to the upper electrode and the lower electrode (Prior Art 3). For example, Patent Document 1 discloses the relationship between the electromechanical coupling coefficient and the ratio of the film thickness of the piezoelectric film to the sum of the film thicknesses of the upper electrode and the lower electrode in a case where the upper electrode and the lower electrode are made of tungsten, aluminum, gold, or copper.
In accordance with Prior Art 1, however, it is difficult to form a high-quality thin film of PZT or PbTiO3, and practical resonant characteristics cannot be obtained. In accordance with Prior Art 2, to obtain a piezoelectric film having a suitable orientation for an increase in the electromechanical coupling coefficient, a number of control and management operations are required to control and manage the material for the base of the piezoelectric film, the surface roughness of the base, and the conditions for forming the piezoelectric film. If those control and management operations are inadequate, the orientation might become unstable in the wafer plane and between lots, resulting in unstable resonant characteristics. In accordance with Prior Art 3, to increase the electromechanical coupling coefficient, the ratio of the film thickness of the piezoelectric film to the sum of the film thicknesses of the upper electrode and the lower electrode needs to be low. However, as the upper electrode and the lower electrode become thinner, the electrode resistance increases, resulting in tradeoff for insertion loss.